1. Field of the Invention
The present invention relates to Surface Mounting Technology (SMT), and more particularly to a surface mounting structure applied to a ball grid array (BGA) that prevents soldering failures in four corners of a chip substrate.
2. Related Art of the Invention
Nowadays, semiconductor chips are becoming thinner and smaller in size, yet higher processing speeds are required. SMT (Surface Mounting Technology) for a BGA (Ball Grid Array) is well developed as a result.
However, a result of the actual implementation of the surface mounting process in the prior art, a concentration of heat occurs in four corners of a chip substrate. Solder balls in the four corners therefore need to be improved with better heat dissipation. A conventional method of enhancing heat dissipation is to add an extension portion outwards from the soldering pad to increase the heat-dissipation surface of the soldering pad. Yet an extension portion extending from the soldering pad creates a smaller lateral contact surface between the solder ball and the soldering pad. Meanwhile the four corners of the substrate develop wraps under high temperature. This causes extra stresses on the solder balls, so that the solder balls fail to perfectly solder with the soldering pads in the four corners of the substrate. In view of the problem, several solutions in the prior are proposed.
In R.O.C (Taiwan) Patent No. 461053, a metal enhancing layer is applied to those portions surrounding the under surface of the substrate to strengthen the substrate and prevent the substrate from being wrapped under high temperature. However, such a solution increases the complexity and cost of the manufacturing process.
In P.R.C. (China) Patent No. CN 1742371A, virtual balls without electrical connections are applied to the four corners of the BGA package to increase SMT soldering reliability. However, such a solution wastes space on the chip.
In U.S. Pat. No. 6,927,491B1, larger solder balls are applied to the four corners of the BGA package structure, thereby increasing SMT soldering reliability However, such a solution also incurs the problem of wasting space on the chip.
Other approaches to increasing the SMT soldering reliability in the four corners of the BGA package include applying glue to the four corners of the BGA package, or adding a metal frame surrounding the BGA package. However, these solutions also increase the complexity either cost of the manufacturing process or troubleshooting.
The aforesaid technologies in the prior art all address the soldering failure problem of the four corners during the SMT manufacturing processes. However, those solutions incur problems including increasing the complexity and cost of manufacturing processes, and wasting space on the chip.